Hydraulic oscillator

ABSTRACT

The disclosure concerns a hydraulic oscillator of the type in which the distributing valve for the power element is shifted by pressures developed by pumping members operated by the power element. The device is relatively simple and is truly selfstarting regardless of the position in which the power element is resting. Preferably, the distributing valve acts as a flow divider which diverts a portion of the supply flow to tank, and thereby precludes development of large pressure peaks at the ends of the strokes of the power element. Moreover, the flow-dividing action is adjustable so that oscillator frequency can be selected independently of the rate of supply.

United States Patent 1191 Fruehauf Jan. 22, 1974 [54] HYDRAULIC OSCILLATOR FOREIGN PATENTSOR APPLICATIONS [751 Invenm x9 3 Fruehauf Kalamazoo 784,684 10 1957 Great Britain 91 317 [73] Assignee: General Signal Corporation, P Maslousky Rochester, Attorney, Agent, or Fzrm-Jeffrey S. Medmck I Filed: May 15, 1972 ABSTRACT [21] Appl. No.: 253,514 The disclosure concerns a hydraulic oscillator of the type in which the distributing valve for the power ele- 1 ment is shifted by pressures developed by pumping [52] US. Cl 91/284, 91 I317, 99110332360, members operated y the power element. The device [51] Int Cl F01] 25/02 F01] 31/00 is relatively simple and is truly self-starting regardless of the position in which the power element is resting. [58] meld of Search V 326 Preferably, the distributing valve acts as a flow divider [56] References Cited which diverts a portion of the supply flow to tank, and thereby precludes development of large pressure UNITED STATES PATENTS peaks at the ends of the strokes of the power element. 1,057,701 4/1913 Bayles 91/3l7 Moreover, the flow dividing action is adjustable so f g 1 that oscillator frequency can be selected indepen- 2932175 4/1960 I: 911317 demly of the rate of 2,970,579 2/1961 Paris 91/317 10 Claims, 4 Drawing Figures ss\ 57 T1 1 58 1 49 t 37 9 1 I A 22 Q3 A a $6 433.27:- 52 1, l 1" L 1f L: 52 \j a I\\ x i 1 I 1 m 1/ I n J 1/ 1 Q L5 7"? H L f minimum 22 m4 sum 2 or 2 HYDRAULIC OSCILLATOR BACKGROUND AND SUMMARY OF THE INVENTION The prior art discloses various hydraulic oscillators of the type in which the distributing valve for the power element is shifted by amplified pressures developed by pumping members operated by the power element itself. Some examples of this type of mechanism are shown in the following U.S. Pat. Nos.

2,970,579 granted Feb. 7, 1971 3,007,452 granted Nov. 7, 1961 3,150,566 granted Sept. 29, 1964 3,540,348 granted Nov. 17, 1970 These prior oscillators, however, are rather complex, and therefore expensive, and either are not self-starting under all conditions or require auxiliary mechanical apparatus in order to afford that capability.

The primary object of this invention is to provide an improved oscillator of the kind mentioned which is relatively inexpensive to manufacture, and in which the distributing valve is operated exclusively by hydraulic pressures and is self-starting under all conditions. According to the invention, the new oscillator employs a unified, four-way distributing valve which is urged toward one of its two operative positions by a spring or other suitable biasing device, and is shifted by a pair of fluid pressure motors which act on the valve in opposition to each other and have unequal effective areas. The smaller actuating motor, which acts on the valve in the same sense as the biasing device, normally is connected with one working space of the power element through a booster pumping device, but receives fluid at the amplified output pressure of that device during the terminal movement of the power element in one direction. Similarly, the larger actuating motor normally communicates with the other working space of the power element, but is furnished with fluid at the amplified output pressure of a second booster device during terminal movement of the power element in the opposite direction. In addition, the two actuating motors are interconnected through a restricted passage. As will be apparent from the detailed description which follows, this organization of parts affords self-starting capability under all operating conditions and results in an inherently simple and relatively inexpensive oscillator.

In the preferred form of the new oscillator, the distributing valve also performs a flow-dividing function and diverts to a reservoir or tank a controlled portion of the fluid supplied to the oscillator. This feature precludes full cut-off of the supply flow at the ends of the strokes of the power element, and thereby minimizes pressure peaks or shocks in the supply system. Moreover, in accordance with a further refinement, the flowsplitting action of the distributing valve can be adjusted. Inclusion of this feature allows oscillator frequency to be selected independently of the rate of supply, and thereby makes possible the use of a single oscillator design in various applications even though the rates of supply are quite different. Another feature of the preferred oscillator consists in making adjustable the restriction in the cross-connection between the actuating motors. This expedient is useful to prevent development of excessive pressures in the booster output circuits and to permit control over the rate at which the boosters decelerate the power element.

BRIEF DESCRIPTION OFTI-IE DRAWINGS Several embodiments of the invention are described herein with reference to the accompanying drawings in which:

FIG. 1 is a schematic diagram of the basic oscillator.

FIG. 2 is a sectional view of the preferred oscillator.

FIG. 3 is a sectional view taken on line 3-3 of FIG. 2.

FIG. 4 is a sectional view of an alternative distributing valve for the embodiment of FIG. 2.

DESCRIPTION OF ILLUSTRATED EMBODIMENTS illustrated position, in which it connects spaces 14 and 15 with supply pump 19 and tank 21, respectively, by a biasing spring 22 and a small actuating motor 23, and is shifted to its other operative position, to reverse the connections to spaces 14 and 15, by a largeractuating motor 24. The motors 23 and 24 are interconnected by a passage 25 containing a flow restriction 26, and each also is connected with the transfer port 27 or 28 of a booster pumping unit 29 or 31. Pumping unit 29 comprises a booster cylinder 32 which is aligned with cylinder 12 and opens into working space 14, and a cooperating pumping member comprising a booster piston 33 which is fixed to power piston 13 and fits cylinder 12 with a radial clearance on the order to 0.001 to 0.002 inches. For the major portion of the cycle of piston 13, booster cylinder 32 is in free communication with working space 14, so actuating motor 23 is subjected to substantially the same pressure as that prevailing in the working space. However, as power piston 13 approaches the limit of its travel to the left, piston 33 enters booster cylinder 32 and isolates it from free communication with space 14. Thereafter, leftward movement of piston 13 causes piston 33 to displace fluid from cylinder 32 and discharge it through port 27 at a pressure which is a function of the ratio of the effective areas of pistons 13 and 33. Thus, it will be realized that booster piston 33 serves as both a restricting element and a pumping element.

Pumping unit 31 is identical to unit 29 and includes a booster cylinder 35 and a cooperating pumping member comprising a booster piston 35. The pumping unit 31 is associated with working space 15 and is effective to pressurize actuating motor 24 during the terminal portion of the rightward movement of power piston 13.

It might be noted here that, in addition to their pump- I ing functions, the units 29 and 31 cooperate with restricted passage 25 to define hydraulic'snubbers for for power piston 13 which bring the latter to a cushioned stop in both directions of movement. Thus, in normal operation, metal-to-metal contact between the stationary and moving parts of motor 11 is avoided.

It will also be helpful to observe that the clearance space between each booster piston 33 and 35 and its associated booster cylinder 32 or 34 constitutes an annular orifice of variable length which performs two different functions during the time power piston 13 comes to rest and reverses direction. During the deceleration period, i.e., as a booster piston moves into its cylinder, the annular orifice restricts flow from the cylinder to the adjacent working space 14 or 15, and thereby permits development of the amplified pressure required to effect shifting of distributing valve 16. On the other hand, as the power piston accelerates at the commencement of the following stroke, the annular orifice permits fluid transfer from the adjacent working space to the booster cylinder, and thus serves to maintain in the associated actuating motor 23 or 24 a pressure sufficient to insure that valve 16 will be maintained in its new position.

When pump 19 is at rest, or the supply of fluid to the oscillator is otherwise interrupted, spring 22 holds valve 16 in the illustrated position. Therefore, as soon as the supply of fluid is resumed, working spaces 14 and immediately will be pressurized and vented, respectively. If, at this instant, piston 13 is in an intermediate position, such as the one shown in FIG. 1, it will commence to move to the right. During the initial phase of this movement, a portion of the oil delivered to working space 14 escapes to tank 21 through a pilot path comprising booster cylinder 32, port 27, passage 25, port 28, booster cylinder 34, working space 15 and port 18. Because of the presence of restriction 26, this pilot flow develops in motor 23 a backpressure of a magnitude sufficient to enable it to overpower the larger motor 24 and to maintain valve 16 in the illustrated position.

As power piston 13 approaches the end of its rightward stroke, booster piston 35 enters cylinder 34 to thereby isolate the cylinder from free communication with working space 15 and develop therein an amplifled pressure which is approximately equal to-the product of the pressure in working space 14 and the ratio of the effective area of piston 13 to the effective area of piston 35. Oil displaced from cylinder 34 is delivered to working space 14 via port 28, passage 25, port 27, and booster cylinder 32, so now restriction 26 causes actuating motor 24 to be subjected to a higher pressure than motor 23. This pressure differential, coupled with the differential between the effective areas of the two motors, enables motor 24 to shift valve 16 to its other operative position and thereby reverse the connections between pump 19 and tank 21 and the working spaces. During the preceding operations, power piston 13 is brought to a cushioned stop by booster unit 31. Therefore, since working spaces 14 and 15 are now vented and pressurized, respectively, power piston 13 commences to move to the left.

During the initial portion of the leftward stroke of power piston 13, the pressure in motor 24 decreases first, because fluid can escape from the motor to tank 21 through passage 25 and booster unit 29, and second, because the withdrawal movement of booster piston 35 increases the volume of that portion of cylinder 34 which communicates with motor 24. However, since the velocity of piston 13 is relatively low at this time, and the length of the annular orifice defined by'piston 35 and cylinder 34 is decreasing, fluid under pressure will be transferred from working space 15 to cylinder 34 through the annular orifice at a rate sufficient to maintain a low, but positive, pressure in motor 24. Since motor 23 is vented to tank 21, this low pressure will be adequate to enable motor 24 to hold valve 16 in its second position. Of course, as soon as piston 35 moves out of cylinder 34, motor 24 will be in free communication with working space 15. Therefore, from this point onward, motor 24 will be subjected to an even higher pressure determined by the load on power piston 13 and the size of restriction 26.

As power piston 13 nears the end of its leftward stroke, booster piston 33 enters cylinder 32 to thereby bring the power piston to a cushioned stop and displace oil through port 27 at the amplified pressure mentioned earlier. This oil flows to working space 15 via passage 25, port 28, and booster cylinder 34, and consequently restriction 26 subjects motor 23 to a pressure higher than the supply pressure prevailing in motor 24. The pressure difference is sufficient to offset the area differential between the two motors, so valve 16 now is shifted back to its illustrated position. This action has the effect of pressurizing working space 14 and venting working space 15, and therefore power piston 13 will now commence to move to the right.

During the short time-period in which piston 13 reverses direction and booster piston 33 is withdrawn from cylinder 32, the pressure in actuating motor 23 will decrease to a low value determined by the rate at which oil can be delivered to it through the annular orifice defined by piston 33 and cylinder 32. Since motor 24 is vented to tank 21 as soon as valve 16 returns to its illustrated position, the combined forces developed by motor 23 and spring 22 will be sufficient to hold the valve in that position.

The power piston 13 will continue to oscillate in the manner described as long as the oscillator continues to receive oil from pump 19. The frequency of the oscillator obviously will be determined by the rate at which oil is supplied.

If the power piston 13 is at rest in a position, such as the one shown in FIG. 2, in which it is at or very near the rightward limit of its motion, the introduction of fluid under pressure into working space 14 either will produce no rightward movement of power piston 13, or will produce so little movement that booster unit 31 will not displace sufficient oil to enable motor 24 to shift valve 16. In this case, motor 24 is pressurized by oil which is delivered to it from working space 14 via booster cylinder 32, port 27 and passage 25. Although some of this oil can escape to tank 21 via port 28, the annular orifice defined by booster piston 35 and cylinder 34, working space 15 and port 18, the restriction to flow through the annular orifice is high enough to maintain in motor 24 a pressure sufficient, in view of the difference between the effective areas of the actuating motors, to enable motor 24 to shift valve 16 to its second position against the opposition of spring 22 and motor 23. Once the valve shifts, oil will be supplied to working space 15, space 14 will be vented to work tank 21, and power piston 13 will commence to move to the left. Venting of working space 14 dissipates the pressure in motor 23, so now motor 24 need only overcome the bias of spring 22 in order to maintain valve 16 in its second position. The actuating. pressure required for this is developed by the oil under supply pressure which flows from working space 15 to motor 24 through the annular orifice in booster unit 31. As the power piston 13 moves to the left, the length of the annular orifice gradually decreases, and consequently so too does the restriction to flow through it. Therefore, the pressure in motor 24 gradually rises to the maximum level which is achieved when piston 35 withdraws from cylinder 34 and re-establishes free flow from space 15 to this cylinder. From this point on, the oscillator will cycle in the manner described earlier.

In the event power piston 13 is at rest in the extreme left-hand position, starting of the oscillator presents no difficulty whatever because valve 16 is biased to the position which effects rightward movement of the piston.

In light of the foregoing discussion, it should now be clear that the improved oscillator is truly self-starting in all rest positions of power piston 13.

It should be observed that, at the end of each stroke of power piston 13, flow from the supply pump 19 is momentarily interrupted. This is an undesirable characteristic because it subjects the pump and the supply system to repeated pressure peaks or shocks throughout the period that the oscillator is in operation. Moreover, as mentioned above, the frequency of oscillation is directly dependent upon the rate of supply of oil from pump 19. This characteristic can also be a disadvantage. For example, the oscillator may be used to operate sickle bar cutters which are towed by tractors of different sizes. Since large tractors usually employ larger hydraulic pumps than small tractors, but the optimum speed for the cutter is independent of its length, the oscillator would have to be made in various sizes in order to cover the whole range of implement sizes. This, of course, increases manufacturing costs. Both of the aforementioned disadvantages are eliminated by the preferred oscillator shown in FIG. 2.

In the FIG. 2 embodiment, the distributing valve 16 also serves as a flow divider whose proportioning action can be varied. The valve comprises a bore 36 which is provided with a central supply port 37, a pair of exhaust or return ports 38 and 39, and a pair of motor ports 41 and 42, and which contains a reciprocable valve spool 43 formed with two axially spaced peripheral grooves 44 and 45 which define three lands 46, 47 and 48. The two operative positions of spool 43 are defined by a pair of stop members 49 and 51 which are screwed into the casing of the oscillator and are held in their selected positions by nylon thread locks 52.

In the illustrated operative position of spool 43, motor port 41 is isolated from return port 38 by land 46 and is connected in free communication with supply port 37 by groove 44, and motor port 42 is in free communication with return port 39 through groove 45 and is in restricted communication with supply port 37 across the right edge 53 of land 47. Thus, only a portion of the oil supplied to port 37 by pump 19 will be delivered to working space 14, and the balance will be diverted to tank 21 through groove 45 and port 39. The quantity so diverted depends upon the position of stop 51, since this setting determines the area of port 37 which communicates with groove 45. On the other hand, in the second operative position of spool 43, motor port 42 is isolated from return port 39 and is in free communication with supply port 37 through groove 45, and motor port 41 is in free communication with return port 38 via groove 44 and is in restricted communication with port 37 across the left edge 54 of land 47. Therefore, as before, the supply stream is split between tank 21 and the expanding working space of motor 11 (in this case, space 15), and the quantity of oil diverted to the tank is determined by the setting of the applicable limit stop (namely, stop 49). The two stops 49 and 51 are set while the oscillator is in operation so that power piston 13 moves at substantially the same velocity in both directions, and that the valve 16 diverts to tank that portion of the incoming flow necessary to produce the desired frequency of oscillation.

It also will be noticed that the restriction 26 in the in terconnection 25 between the valve-actuating motors 23 and 24 in FIG. 2 takes the form of a plug 55 containing three orifices 56, 57 and 58 of different sizes. The plug is rotated by a knob 59 to any one of three positions determined by a detent 61 in order to interpose the selected orifice in passage 25. The adjustability of restriction 26 is a desirable feature in a commercial oscillator because it affords control over the peak snubbing pressure and the rate of deceleration of power piston 13 afforded by the booster units 29 and 31, and thus enables a single oscillator design to actuate loads of different magnitudes.

The adjustable flow-splitting action which characterizes the distributing valve of the FIG. 2 embodiment also is afforded by the alternative version of valve 16 shown in FIG. 4. The spool 143 used in this valve differs from its FIG. 2 counterpart in that it employs a central land 147 whose opposed edges 153 and 154 lie in planes inclined in opposite directions with respect to the longitudinal axis of the spool. Thus, in this case, the rotary position of the spool, rather than the axial positions of its limit stops 149 and 151, determines the degree of restriction in the diversion paths leading from supply port 137 to return ports 138 and 139 in the two operative positions of the spool. The rotary position of spool 143 is varied by a knob 62 which is connected with the spool through a torque-transmitting connection comprising stop 151, slot 63 and cooperating pin 64, the piston of actuating motor 24, and a pin 65 whose ends are received in aligned bores extending through the hollow land 148 of the valve spool. A detent 66, which cooperates with a circumferential series of holes in knob 62, serves to yieldingly hold the valve spool in the selected rotary position. The angles of inclination of edges 153 and 154 are equal, and therefore manipulation of knob 62 simultaneously effects the same change in the flow-splitting action afi'orded in both operative positions of valve 16. Aside from this, the FIG. 4 valve operates in the same way as its FIG. 2 counterpart.

I claim:

1. A hydraulic oscillator comprising a. a double-acting motor having a power element movable in a first direction under the action of fluid pressure in one working space and in a second direction under the action of fluid pressure in a second, opposed working space;

b. a four-way distributing valve movable between a first position in which it establishes a supply path leading from an inlet connection to said one working space and a return path leading from said second working space to an exhaust connection, and a second position in which it reverses the connections between said paths and working spaces;

c. means biasing the distributing valve towards the first position;

d. a pair of valve-actuating motors having unequal effective areas and acting on the distributing valve in oppositon to each other, the larger motor serving to shift the valve toward the second position;

e. first booster pumping means including a first booster cylinder connected with the second working space and with the larger valve-actuating motor, a first pumping member operated by the power element for restricting the connection between the first booster cylinder and the second working space during a terminal portion of movement of the power element in said first direction for forcing fluid from the first booster cylinder to the larger valve-actuating motor during said terminal portion of movement; second booster pumping means including a second booster cylinder connected with the first working space and with the smaller valve-actuating motor, a second pumping member operated by the power element for restricting the connection between the second booster cylinder and the first working space during a terminal portion of movement of the power element in said second direction for forcing fluid from the second booster cylinder to the smaller valve-actuating motor during said terminal portion of movement; and

g. a restricted transfer passage connecting the booster pumping means with the larger actuating motor.

2. A hydraulic oscillator as defined in claim 1 in which the distributing valve also establishes a restricted diversion path leading from the supply connection to the exhaust connection in each of said first and second positions.

3. A hydraulic oscillator as defined in claim 2 in which the distributing valve maintains said diversion path open, but reduces the restriction to flow through it, as it moves between said first and second positions.

4. A hydraulic oscillator as defined in claim 2 including adjustable means for varying the restriction to flow through the diversion path.

5. A hydraulic oscillator as defined in claim 1 in which the distributing valve comprises a. a bore intersected by five axially spaced passages,

there being a central passage which constitutes said supply connection, a pair of end passages which constitute said exhaust connection, and an intermediate passage located between the central passage and each end passage and connected with one or the other of the working spaces; and

b. a spool reciprocable in the bore and formed with two axially spaced peripheral grooves which define a central land and a pair of end lands,

c. the parts of the distributing valve being so arranged that,

' l. in said first position, the first intermediate passage is isolated from free communication with the adjacent end passage by one end land and is in unrestricted communication with the central passage through one peripheral groove, and the second intermediate passage is in unrestricted communication with the adjacent end passage through the other peripheral groove and is in restricted communication with the central passage across one edge of the central land, and 2. in said second position, the first intermediate passage is in unrestricted communication with the adjacent end passage through one groove and is in restricted communication with the central passage of the central land, and the second intermediate passage is isolated from free communication with the adjacent end passage by the other end land and is in unrestricted communication with the central passage through the other peripheral groove.

6. A hydraulic osciallator as defined in claim 5 in which a. the edges at opposite sides of the central land lie in planes normal to the longitudinal axis of the spool; and

b. the distributing valve includes adjustable stops which cooperate with the spool to define said first and second positions,

0. adjustment of one stop serving to shift said first position axially relatively to the passages and thereby vary the restriction to flow from the central passage to the second intermediate passage across said one edge of the central land, and

d. adjustment of the other stop serving to shift said second position axially relatively to the passages and thereby vary the restriction to flow from the central passage to the first intermediate passage across said opposite edge of the central land, and

7. A hydraulic oscillator as defined in claim 5 in which a. the edges at opposite sides of the central land lie in planes which are inclined in opposite directions with respect to the longitudinal axis of the spool; and

b. distributing valve includes adjustment means for rotating the spool about its longitudinal axis and for locking it in a selected rotary position,

c. whereby the restriction to flow from the central passage to the second intermediate passage in said first position and the restriction to flow from the central passage to the first intermediate passage in said second position can be varied in unison.

8. A hydraulic oscillator as defined in claim 1 which includes means for varying the restriction in flow through the transfer passage.

9. A hydraulic oscillator as defined in claim 8 in which the transfer passage is restricted by a transverse orifice member which is pierced by a plurality of bores of different diameters, and which is movable to predetermined positions in each which flow through the transfer passage is restricted by a different bore.

10. A hydraulic oscillator as defined in claim 1 in which a. the double-acting motor includes a power cylinder containing a conforming reciprocable power piston which divides it into said opposed working spaces; and

b. each booster pumping means comprises a booster cylinder which is aligned with and has a smaller diameter than the power cylinder, and a booster piston carried by the power piston and which serves as the pumping member of the booster means P p g,

c. the effective areas of the two booster pistons being determined radial clearance. 

1. A hydraulic oscillator comprising a. a double-acting motor having a power element movable in a first direction under the action of fluid pressure in one working space and in a second direction under the action of fluid pressure in a second, opposed working space; b. a four-way distributing valve movable between a first position in which it establishes a supply path leading from an inlet connection to said one working space and a return path leading from said second working space to an exhaust connection, and a second position in which it reverses the connections between said paths and working spaces; c. means biasing the distributing valve towards the first position; d. a pair of valve-actuating motors having unequal effective areas and acting on the distributing valve in oppositon to each other, the larger motor serving to shift the valve toward the second position; e. first booster pumping means including a first booster cylinder connected with the second working space and with the larger valve-actuating motor, a first pumping member operated by the power element for restricting the connection between the first booster cylinder and the second working space during a terminal portion of movement of the power element in said first direction for forcing fluid from the first booster cylinder to the larger valve-actuating motor during said terminal portion of movement; f. second booster pumping means including a second booster cylinder connected with the first working space and with the smaller valve-actuating motor, a second pumping member operated by the power element for restricting the connection between the second booster cylinder and the first working space during a terminal portion of movement of the power element in said second direction for forcing fluid from the second booster cylinder to the smaller valve-actuating motor during said terminal portion of movement; and g. a restricted transfer passage connecting the booster pumping means with the larger actuating motor.
 2. A hydraulic oscillator as defined in claim 1 in which the distributing valve also establishes a restricted diversion path leading from the supply connection to the exhaust connection in each of said first and second positions.
 2. in said second position, the first intermediate passage is in unrestricted communication with the adjacent end passage through one groove and is in restricted communication with the central passage of the central land, and the second intermediate passage is isolated from free communication with the adjacent end passage by the other end land and is in unrestricted communication with the central passage through the other peripheral groove.
 3. A hydraulic oscillator as defined in claim 2 in which the distributing valve maintains said diversion path open, but reduces the restriction to flow through it, as it moves between said first and second positions.
 4. A hydraulic oscillator as defined in claim 2 including adjustable means for varying the restriction to flow through the diversion path.
 5. A hydraulic oscillator as defined in claim 1 in which the distributing valve comprises a. a bore intersected by five axially spaced passages, there being a central passage which constitutes said supply connection, a pair of end passages whiCh constitute said exhaust connection, and an intermediate passage located between the central passage and each end passage and connected with one or the other of the working spaces; and b. a spool reciprocable in the bore and formed with two axially spaced peripheral grooves which define a central land and a pair of end lands, c. the parts of the distributing valve being so arranged that,
 6. A hydraulic osciallator as defined in claim 5 in which a. the edges at opposite sides of the central land lie in planes normal to the longitudinal axis of the spool; and b. the distributing valve includes adjustable stops which cooperate with the spool to define said first and second positions, c. adjustment of one stop serving to shift said first position axially relatively to the passages and thereby vary the restriction to flow from the central passage to the second intermediate passage across said one edge of the central land, and d. adjustment of the other stop serving to shift said second position axially relatively to the passages and thereby vary the restriction to flow from the central passage to the first intermediate passage across said opposite edge of the central land, and
 7. A hydraulic oscillator as defined in claim 5 in which a. the edges at opposite sides of the central land lie in planes which are inclined in opposite directions with respect to the longitudinal axis of the spool; and b. distributing valve includes adjustment means for rotating the spool about its longitudinal axis and for locking it in a selected rotary position, c. whereby the restriction to flow from the central passage to the second intermediate passage in said first position and the restriction to flow from the central passage to the first intermediate passage in said second position can be varied in unison.
 8. A hydraulic oscillator as defined in claim 1 which includes means for varying the restriction in flow through the transfer passage.
 9. A hydraulic oscillator as defined in claim 8 in which the transfer passage is restricted by a transverse orifice member which is pierced by a plurality of bores of different diameters, and which is movable to predetermined positions in each which flow through the transfer passage is restricted by a different bore.
 10. A hydraulic oscillator as defined in claim 1 in which a. the double-acting motor includes a power cylinder containing a conforming reciprocable power piston which divides it into said opposed working spaces; and b. each booster pumping means comprises a booster cylinder which is aligned with and has a smaller diameter than the power cylinder, and a booster piston carried by the power piston and which serves as the pumping member of the booster means pumping, c. the effective areas of the two booster pistons being equal, and each is sized to fit its cylinder with a predetermined radial clearance. 